The measurements were made in a cylindrical Rayleigh-Bénard convection cell, which has been described in detail previously [1]}, [2]}, [3]}.
Here we give only its essential features. The top and bottom conducting plates are made of pure copper with a thin layer of nickel to avoid oxidation. The sidewall is made of Plexiglas. To avoid distortions in the images viewed by the camera, a square-shaped jacket is fitted around the sidewall of the convection cell. As shown in figure REF (b), the jacket is filled with water. The diameter and height of the cell is \(D=19.6\)  cm and \(H=18.6\)  cm, respectively. The aspect ratio \(\Gamma = D/H\)  is thus close to 1. Two (three) thermistors are embedded in the top (bottom) plate. The top plate temperature is maintained constant by a refrigerated circulator (Polyscience Model 9702) that has a temperature stability of \(0.01^{o}C\) . A NiChrome wire (26 Gauge, Aerocon Systems) surrounded by fiberglass sleeving and Teflon tape is distributed inside the grooves carved under the bottom plate. The wire is connected with five DC power supplies (GE Model GPS-3030) in series to provide constant and uniform heating. During the measurement, the whole cell is placed in a homemade thermostat box that is kept at the same temperature (\(30^{o}C\) ) as that of the fluid at the centre of the cell. During the experiment the cell was tilted by an angle \(\theta \)  such that the circulation plane of the LSC was parallel to the image plane of the camera (the \(x\) -\(z\)  plane, see figure 1).
